1. Field of the Invention
The present invention relates to a projection optical system, an illumination apparatus provided with this projection optical system as an illumination optical system, and an exposure apparatus loaded with this illumination apparatus. In particular, the present invention relates to an optical system used for projecting an image of an object surface onto an image surface while magnifying or reducing it in its longitudinal and lateral directions with magnifications different from each other, an illumination apparatus provided with this optical system, and an exposure apparatus loaded with this illumination apparatus.
2. Related Background Art
In conventional exposure apparatuses provided with projection optical systems, excimer laser and the like have been used as their light source. Since a parallel beam of light emitted from the excimer laser or the like has a rectangular cross-sectional shape, it forms images at positions in longitudinal and lateral directions different from each other by way of a projection optical system. Also, since angular shifts are likely to occur in the laser beam emitted from the light source, axial astigmatism may be generated.
Here, a projection exposure apparatus for manufacturing semiconductor devices, liquid display devices, and the like is a considerably large apparatus as a whole, thereby necessitating a large floor space for its installation. As the light source in the projection exposure apparatus, in general, mercury lamp, laser light source, or the like is used. In particular, a laser light source such as excimer laser light source is a light-emitting apparatus which is much larger than a light source such as mercury lamp.
Accordingly, in particular, in a projection exposure apparatus using a laser light source such as excimer laser light source, it is necessary for the laser light source to be disposed separately from the main body of the projection exposure apparatus. As a result, the use of the laser light source increases the floor space needed for the projection exposure apparatus as a whole.
Therefore, in order to position the laser light source apparatus and the main body of the projection exposure apparatus as close as possible to each other, the beam of light should be drawn around as short as possible in the optical path between the laser light source and the main body of the projection exposure apparatus. Alternatively, a deflecting member such as a reflective mirror should be appropriately disposed in the above-mentioned optical path so as to repeatedly bend the beam of light in a skillful manner.
In general, the projection exposure apparatus uses an optical integrator in order to uniformly illuminate a mask in a superposing manner. Accordingly, this optical integrator must be illuminated efficiently. When the laser light source apparatus and the main body of the projection exposure apparatus are disposed separately as mentioned above, however, the optical path between the laser light source and the optical integrator tends to become very long.
Consequently, due to the initial positional deviation of the excimer laser light source, which is a huge light-emitting apparatus, or the positional deviation of optical members caused by vibration during exposure, for example, the optical integrator may not be illuminated efficiently.
In a configuration in which a relay lens system is disposed between the laser light source and the optical integrator, various kinds of optical members such as cylinder beam expander should be disposed between the laser light source and the relay lens system or between the relay lens system and the optical integrator. Accordingly, a deflecting member such as a reflective mirror should be disposed in the optical path on the converging side of the relay lens system so as to bend the beam of light. In this case, depending on the position at which the deflecting member is disposed, the energy density of the incident laser beam becomes so large that the deflecting member may be damaged.